1. Field of the Invention
The present invention relates to method of starting and stopping a methanol reforming apparatus that generates a hydrogen enriched gas from water and methanol, and an apparatus for supplying a fuel to said methanol reforming apparatus.
2. Description of Related Art
Fuel cells have been developed as a means for driving low-pollution vehicles and for supplying electric power to vehicles.
Hydrogen in the form of a compressed hydrogen gas or liquid hydrogen is convenient as the energy source for the fuel cell, but there are problems regarding the ease of handling. Thus there is a demand for a hydrogen supply apparatus which is very easy to handle.
Recently, technologies for preparing hydrogen enriched gas by reforming alcohol or hydrocarbons using a catalyst have been intensively studied and developed, and various catalysts and reaction apparatuses have been invented.
An example of the reaction apparatus is a methanol reforming apparatus (hereinafter referred to as "reformer") 1 shown in FIG. 6. For supplying fuel to the reformer 1, methods are known such as separately providing a water tank and a methanol tank or separately providing a mixed water-methanol solution tank 2 and a methanol tank 3 in order to prevent water from freezing in cold climates, as disclosed in Japanese Patent Application, First Publication No. Hei 8-91804, wherein water and methanol are delivered from the two tanks in a liquid state to an evaporator 4 to produce a mixed water-methanol gas which is then supplied to the reformer 1.
However, when methanol, which has a low flash point and a low ignition point, is reformed, particularly when employing the autothermal reaction process, wherein a partial oxidization reaction and steam reforming reaction are carried out at the same time, methanol vapor and air coexist on a catalyst that has a high temperature, and therefore the reforming process must be carried out in a strictly controlled system to prevent the reaction from proceeding at an excessively high rate.
Specifically, the mixing ratio of water, methanol and air must not be within a range in which the reaction proceeds at an excessively high rate (hereinafter this range is referred to as "high-rate reaction region"), and the amounts of these materials to be introduced must be strictly controlled.
Before starting the operation of the reformer, on the other hand, the reformer must be warmed and particularly the catalyst layer must be warmed by a heating means until the catalyst becomes active. Hot air or electric heating have been normally used for this purpose.
While water, methanol and air are introduced into the catalyst after the warm-up operation, it is very difficult to control the mixing ratio of the three components so as to avoid the high-rate reaction region. To get around this difficulty, such measures have been taken in the prior art as changing the order of introducing the materials, for example, introducing air after water and methanol have been introduced.
In practice, such measures involve the problems that it takes a long time to start the operation or that special means are required to warm up the catalyst.
To prevent water from freezing in a cold climate, it is more advantageous to provide a mixed water-methanol solution tank 2 than to separately provide a water tank and a methanol tank. Actually, however, it is a common practice to provide a methanol tank 3 in addition to the mixed water-methanol solution tank 2 and to control the mixing ratio using the methanol tank 3 in order to obtain the desired ratio of water and methanol.
As a consequence, there was a problem in that a mixed water-methanol gas having a ratio outside of the high-rate reaction region cannot be immediately supplied during the starting/stopping operations of the reformer 1, when the control tends to be unstable.
A similar problem can also be expected in the case that a water tank and a methanol tank are separately provided.
On the other hand, stopping the operation of the methanol reforming apparatus, the supplies of water, methanol and air are stopped and the catalyst layer is cooled. However, if an excessive amount of air is supplied during the autothermal reaction process, a partial oxidization reaction proceeds, thus giving rise to the possibility of an uncontrolled thermal runaway of the catalyst layer.
Therefore, when stopping the methanol reforming apparatus, it is also important to strictly control the mixing ratio of water, methanol and air so as to avoid the high-rate reaction region. Particularly, since the operation must be stopped while controlling the air supply to a proper level, it takes a long time to stop the operation.
Also, if water and methanol used as the fuel remain in the apparatus when restarting a methanol reforming apparatus that has been stopped, the remaining fuel will deviate the mixing ratio of water, methanol and air when starting the operation, thus giving rise to the possibility of thermal runaway of the catalyst.
Therefore, the operation of stopping the methanol reforming apparatus must be carried out while paying attention to the mixing ratio of water, methanol and air, and the fuel must not be allowed to remain in the apparatus. Thus, the stopping operation takes a long time and requires a complicated control procedure.